Echo suppressor



Det- 10, 1929.

D. MITCHELL ET AL ECHO SUPPRESSOR Filed Aug. 25. 1928 2 Sheets-Sheet Dec. 10, 1929. D. MITCHELL ET A; 1,738,536

EC'HO SUPPRESSOR Filed Aug. 23. 1928 2 Sheets-Sheet 2 I; INVENTORS fl gfimezzwm Sale/e6 AT'ILORNEY f reams bee. in, teas IEGBAEEDE 03 NEW YORK This invention relates to echo suppressors on telephone lines, and more particularly to a method and means by which the sensitivity i of the suppressor may be increased without deleterious efiects from noise.

Another obcontrol of the line to obtain such control if need be; that is, to ermit breaking. We

obtain these results y a special difierential action between the two parts of a suppressor as connected to the opposite lines.

The invention will be better understood from the following 'specificationand the accompanying drawing in which Figure 1 is a circuit diagramof a four-wire telephone circuit with a suppressor showing our invention. Figs. 2, 3 and teach represent modifications of the invention of Fig. 1. V

Referring more particularly to Fig. 1, there is shown the four-wire circuit consisting of the lines L and L the upper one of the figure being adapted to transmit con versation fromwest to east and the other from east to west. These two lines are associated at their terminals E and W with the usual hybrid coils from which lines continue to the or repeaters A. Bridged across the line L preferably adjacent to one of the repeater stations, is a tuned amplifier circuit with adjustable amplification the output of which contains a transformer or other suitable connection to one side of the suppressor. As shown in the figure, the transformer has a primary winding 5 and two secondary windings 6 and 7. Ihe secondary winding 6 leads to the input of a detector circuit comprise ing the detector tube D the output circuit of which contains the. winding 9 of a relay. This detector circuit is supplied with the usual batteries for filament heating and for plate voltage. In addition, it may include a grid biasing battery 10 and in the gridcircuit there is also included a resistance 11 the function of which will be later explained. The relay 9 is taken to represent a single relay or a chain of relays devised to give a slow release in any manner well known in the art.

, subscribers. In general the lines L and L will be provided with a number of amplifiers nennn trimaran, or new rom, AND HAROLD c. srtnn'r. or LAECWONT, new

roan, nssrenons r AMERICAN TELEPHONE AND TELEGRAPH comrAnY, a con;

EGHQ PS'D'PPRESSOB Application area August as, 1928. Serial No. 301,660. 1

Adjacent the line L is a relay 12 which, by suitable contacts, will upon operation ut a short-circuit or other loss across the line%i The relay 12 is supplied with power from the battery 13 through a resistance 14. When the relay 9 is not operated, a ground connection is established through the back contact of the relay which short-circuits the rela 12, thus leaving this in the off position. lipon operation of the relay 9, however, this shortcircuit is removed and the relay 12 may then be operated by the battery 13 to place the loss across the line L A strictly symmetrical arrangement is provided for the lines, looked upon in the reverse direction, there being bridged across the line L the tuned amplifier with the output transformer having coils 6 and 7 '.I The coil 16 is connected to the input of a detector D the grid circuit of which contains the biasing battery 10' and the resistance 11' and the output of which includes the slow release relay or chain of relays 9, all as described pre viously'. There is also provided the relay 12 for placing a short-circuit or loss across the line L and a battery 13' and resistance 14' to perform the same functions as already described.

An interlocking connection is provided between the two parts consisting of the conductor 16 leading from the back contact of the relay 9' to the "front contaotof the relay 9. Similarly, a conductor 16 leads from the back contact of the relay 9 to the front contact of the relay 9. Another cross-connection between-the two parts of the suppressor is provided in that the secondary winding 7 is connected through a rectifier 20 across the resistance 11 in the grid circuit of the detector D Similarly, the secondary winding 7 is connected through a rectifier 20' across the resistance 11 in the grid circuit of the diverted into the tuned amplifier and willbe detected in the detector circuit 1);, op-

erating the relay 9 to remove the shortcircuit from the relay 12 and thus placing a short across the lineL This short or introduction of loss on the line L eliminates the echo effects which it is the purpose of this condenser 21, providing .for a restoration of the circuit to normal after the speech currents haveceased for sutficient time, so that there is no" longer any echo in L 7 It may be that a certain amount of noise current exists on either of the lines, and there may in particular be sufiicient noise on the line L to operate the relay 9' through the detector D In order to overcome this action, while telephone current is coming in from W, the winding 7, with its rectifier 20, is again used for introducing bias on D The larger the amount of noise on the line L the greater must this biasing voltage be,

and this is naturally provided for by the,

subscriber at W speaking more-loudly.- Thus, his control of the circuit, due to the difi'erential action of the apparatus, depends on the volume of his speech currents being greater than the volume of the noise currents. By the selection of the proper apparatus this 'difi'erential action may be obtained so long as the transmission lines are not too noisy to be practical for telephone transmission service. i

The action in the reverse direction isidentically the same and, therefore, does not need to be explained. It should be pointed out, however, that in case E commences talking while W has control of the line he will find his own line L short-circuited, so that his speech will not be transmitted through to W. It will, however, be transmitted through the tuned amplifier circuit to the detector D but in general the biasing efl'ect on the resistance 11 will be suflicient to leave W in control of the line so long as he continues speaking. The arrangement does, however, permit E to break in, for if he speaks with suiiicient loudness he will overcome the bias introduced at 11 and operate the relay 9, thus removingfthe short across the line L; through the releasing of the relay 12 by making contact with the conductor 16. ere again, then, the differential action between the two telephone currents is operative to permit breaking.

One aspect of the circult arrangement of Fig. 1 is that the tuned amplifier bridge of the line L shall be separated from the short circuit controlled by the relay 12. by some suitable devise so that the short circuit Wi not be effective or .completely effective against the bridged tuned amplifier. This may be accomplished by having these two bridges on the opposite sides of a repeater A or by replacing such arepeater by an artificial line of suitable impedance characteristics. We consider this feature of the separation of the two functions of a suppressornamely, that'of putting a loss across the line 1 and that of preventing false operation-to be of importance.

The bridged amplifiers have been spoken of as tuned. This is for the purpose of discriminating against the noise currents which are most likely to be troublesome in such a line. Obviously, the tuning may consist of the usual resonant circuit tuned for an desired frequency, and generally quite or it may consist of a band filter permitting the passage of those voice frequencies which are most effective and which, at the same time, are not too close to those characteristic of the noise disturbances of the line. In addition, the amplification given by these amplifiers is made adjustable to permit of applying the required amount of power to the rectifiers or detectors associated with their outputs.

Fig. 2 shows a circuit which is in most part essentially the same as that of Fig. 1

but, incorporating certain modifications to our invention. This figure shows the lines L and L and the separation of the two functionsof the suppressors 'by the insertion of a repeater or an artificial line, or other equivalent arrangement, between the loss bridge and the tuned amplifier bridge. In this modification the biasing resistances 11 and 11, which in Fig. 1 were in the grid circuit of the detector tubes, have been placed in the grid circuits of the tuned amplifier circuits,

here indicated by C and C. In connection with these biasing resistances there are shown condensers and inductances to filter out the alternatingcurrent component of the'rectified currents in much the same way. as shown for Fig; 1. A second modification in this figure relates to the connections for the relays 12 and 12'. Upon the operation of the relay- 9, the ground -connection for the'relay 12' through the conductor 26' is interrupted, thereby keeping clear the line for conversation from west to east. At the same time, the circuit is closed for the relay 27 which, upon operation, removes the shortcircuiting ground for the relay 12, permitting the operation ofthis relay through a circuit comprising battery, relay 12, conductor 26 to ground through the back contact ofthe relay 9.- It will benoted from this figure that if E speaks-with suificient volume to operate the relay 9', the bridging circuit of the relay 12 is'immediately opened, clearing the line for hisconversation. This fparticular type of interlocking circuit we nd of special advantage. In Fig. 3 still a further modification of our circuit is shown. In this case, E=-.W conversation is impressed on the amplifier fl A and detector D to operate the loss bridge w 30. At, the same time, part of the output of the amplifier A is shunted to suppl a rectifier T causing current to flow throug 1 resistance R which is in the grid circuit of g the detector D The rectified current in R is in such a direction as to put a negative bias on the grid of D thus partially dis- 4 abling that detector. A battery B is so poled that current tends to flow through the resistance R in. the opposite direction. In

View of the rectifier T however, no such.

- current can flow, but it does prevent biasing current from flowing through R in the desired direction unless and until the alter nating current voltage impressed on the rec-' tifier exceeds the voltage of the battery Bi An exactly symmetrical arrangement is provided for telephone currents coming from the opposite direction onto line L In order to obtain proper adjustment for the operations' noted above the biasing'batteries B 'and B should be given a definitely chosen value. Thus, there is always a certain negative bias on the tubes T and T and in order that false operation protection shall be established by these tubes it is necessary'that shall exceed a certain minimum. This minimum is set in accordance with the amount of loss placed on the line by resistances and '31 so that there will be no protection established against false operation until the protection established by these resistances has become inadequate. In this circuit, also, we find it desirable to place in'the grid circuit of D and D biasing batteries. In this modified circuit, as in the others, it will be noted that there is present the diflerential action between the currents on the two lines.

Fig. 4 shows still a further modification. Here E W conversation will be impressed upon the amplifier A with the output of which there is associated a rectifier T and a relay controlling the'bridged loss 30. It will be observed that in this much simplified circuit the functioning of the detectors and rectifiers have been combined. Also, it is to be understood, in this as well as in the three previous circuits, that full wave .rect'ifiers the currents arriving at the tuned amplifier may be desirable in place of the simple onesecho suppressor, the method of eliminating false operation which consists in controllingthe W E sup ressor' from the E W suppressor by the ifi'erential action of the currents in the WE and EW lines.

2. In a telephone circuit comprising an echo suppressor, the methodof eliminating false operation whichfconsists in controlling the vW*E suppressor from the EW sup-v pressor by the excess of telephone current in the EVV line over that in the WE line. 3. In a telephone circuit comprising an echo suppressor, the method. of eliminating,

echo which consists in separating the suppressor functions of introducing loss and-of preventing false operation and of controlling the one side of the suppressor from the other side by the differential action of the currents in the L-E and E'W lines.

4. In a four-wire telephone circuit, an echo suppressor comprising anE+lV and a WE suppressor, and means in the WE suppressor to partially prevent its operation, said means responding in proportion to rent in the E-W line.

5. In a four-wire telephone circuit, an echo suppressor comprising an E-W and a WE suppressor adapted to be operated, respectively, by currents in the E-VV and the VV-E lines, and means in each to partially the ourdisable it, said means responding in proportion to the current in the other line.

6. In a four-wire telephone circuit, an echo suppressor comprising an EVV and a l/VE other is closed.

7 In a four-wire telephone circuit, an echo suppressor comprislng an EVV and a WE suppressor adaptedto be operated respectively by currents in the E--WV and the WE lines, each of these suppressorsincluding a vacuum tube amplifier-detector, means conne cted with each of the latter for partially disabling it, said means. responding in proportion to the current in the other line. 8. t In a four-wire telephone circuit, an echo suppressor comprising an E-VV and a lV--E suppressor adapted to be operated respectively by currents in the EW and the W E lines, each of these including a vacuum tube amplifier-detector, a biasing resistance in a grid circuit ofeach-of the latter, a crossconnection between the two circuits whereby blocking current through the resistance in the WE and in the EW- suppressor will flow in proportion respectively tothe current in the -EW line and in the WE line.

9. In a four-wire telephone circuit, an echo suppressor comprising an E-'-W and a WE suppressor adapted to be operated respectively by currents in the E-W and the WE lines, each .of these includinga vacuum tube amplifier-detector, a baising resistance in a grid circuit of each of the latter; a cross-connection between the two circuits whereby blocking current through the resistance the ,WVE and in' the E-W suppressor will How in proportion respectively to the current in the E'W line and in the W'E line, and an interlocking connection between the EW and the WE suppressors by which one loss bridge is opened when the other is closed.

10. In a four-wire signaling system subject to noise disturbances and provided with echo suppressors,"the method which permits operation at a maximum sensitivity of the echo suppressors, which consists mfcausm each suppressor .to react on the other an reduce the loss in one line and'increase the loss in the other.

. 11.'"In. a four-wire signaling system subject to noise disturbances and provided with an echo suppressor, the method which permits operation at a maximum sensitivity of the suppressor, which consists in reducing the echo in one hne from nolse ori natmg 1n the other, reducing the loss in the t named line and increasing it in the second line when signals come over the first line.

12.. A four-wire signling system subject to noise'disturbances and provided with an echo suppressor, said suppressor comprising means for applying loss of sufiicient magnitude to reduce echo to an unobjectiona'ble I value and separately to prevent false operation.

13. In a telephone circuit including WE and E-eW lines andan echo suppressor associated therewithQhaving W-E and E-W suppressor talements, the method; which con sists in amplifying the telephone energy from the E-VV line to a eater value than that from the. W-E line the EW suppressor'element, and controllingthe -W.E suppressor element from its EW suppressor element by the excess of the ampli ed energy from the E-W line over that from the WE line.

In testimony whereof, we have si na'me'sto this specification this 22n August,192;8.

DOREN MITCHELL. HAROLD C. SILENT.

ed our day of fore impressing it on 

